1. Field of the Invention
The present invention relates to a coating apparatus for applying a liquid to a semiconductor wafer, and more particularly to a spin coating apparatus for applying a thin uniform thickness coating of a photoresist or developer to a semiconductor wafer.
2. Description of the Related Art
As the degree of miniaturization of semiconductor devices increases, the control of the thicknesses of photoresists, developer, etc. becomes increasing critical. Also the prevention of the formation of bubbles is critical. It is conventional to apply coatings on wafers by spraying the liquid resist or developer over a wafer mounted on a spindle as it is rotated. A typical prior art apparatus is illustrated in FIG. 1. A wafer 10 is shown mounted on a rotation spindle 12 as developer solution 14 or other liquid is dropped or sprayed on the wafer from nozzle 16. Line 18 supplies the liquid to the nozzle 16. The temperature of the liquid can be controlled by a suitable heat exchange apparatus, not shown, which maintains the liquid at a uniform desirable temperature to achieve a desired viscosity.
Current design in semiconductor manufacture normally has a nozzle located at an extreme height, which results in a relatively high impact of the liquid onto the product wafer. This results in poor critical dimensions of the coating geometries across the wafer. Also bubble formation, due to the alkaline nature of the developer liquid, is very dominant, and can also cause an impact on the developer uniformity performance. Even if the nozzle is simply moved near to the surface of the substrate or wafer, the problems are not solved due to the high pressure of impact of the solution onto the wafer. The pressure of the development fluid onto the wafer can be as high as 6 psi., because of the needed pressure to dispense the fluid. FIG. 4 shows the effect of the prior art process at curve 80 in the plot of Critical Dimension (CD) versus wafer radius after the development process. It shows the greater development of the photoresist at the center of the wafer than at the sides of the wafer. This is also better understood with reference to FIG. 5 wherein the result of this over development at the center, using a typical 1.0 micrometer feature size results in photoresist mask of the desired 1.0 micrometer lines 84 at the edges of the wafer 10 and the undesired (too narrow) 0.8 micrometer lines at the center of the wafer. It is seen that curve 82, which is the result of the present new apparatus and method as described below would produce uniform mask dimensions across the wafer.